I've read several biographies of Stalin (Stalin: A Biography, Peter Service and Stalin: the Court of the Red Tsar, Simon Sebag Montefiore are the two best researched), and am familiar with the Soviet interwar years through the end of Stalin's life. The initial purges Stalin masterminded to secure power involved faked conspiracies first against the leftists of the party by allying with the rightists, and once the left opposition was destroyed Stalin purged his former rightist allies as well. Once the party was united behind his unquestioned leadership Stalin used further persecutions to enact his domestic political and economic agenda.

Stalin had taken advantage of his position as general secretary of the Party by filling positions within the state and security apparatus with his loyalists, which made it easy for those loyalists to denounce old Bolsheviks who didn't rely on him for their power or position and accuse them of being "Trotskyist", "Zinovievist", or "Bukharinist" plots (i.e. either rightist or leftist deviationists from the accepted party line). This leverage over existing party elites, who leapt over themselves to prove their loyalty in implementing Stalin's agenda, combined with the loss of the party members who had masterminded the revolution and the civil war victory (the old Bolsheviks Stalin eliminated first) left Stalin with a degree of control that would have made opposition suicidal.

After that long and overly simplistic summary, I'll try and answer your question: after Stalin had total control of the party, and had staffed the state and security organs with his fanatical loyalists his next move was to escalate the terror. Either exploiting a successful lone wolf attack, or organizing Kirov's assassination himself, Stalin embarked on a general terror throughout society as a means of reshaping Soviet society and its economy along industrial lines as quickly as possible while blaming the mass repressions he used as tools of this implementation on wreckers, saboteurs, foreign agents and - more sinister still - domestic agents within the very heights of the party plotting its downfall.

So, did they exist? Probably each of those categories named did exist in some numbers. Perhaps during Stalins early reign before the terror reached into every facet of the nation there was even some organized resistance. Certainly the collectivization campaign saw widespread resistance especially from subject nations, and the chaos of the German invasion let loose much of that frustration in nationalist resistance organizations in Ukraine and the Baltic Countries.

Was there enough serious plotting and resistance during the great terror to justify such large scale oppression? In all likelihood: No. The lack of successful or even failed assassination plots proven to have taken place amongst the party leadership, and the (forgive the term) prophylactic approach the security organs employed of rounding up anyone who could potentially be an enemy make it hard to believe in a serious organized resistance such as the party leadership claimed existed during the show trials.

This, however, leads to a further and more intriguing question: did Stalin or the other party bosses believe that level of resistance existed? This is hard to say. It is difficult to divest the cynical political mastermind from the untrusting and brooding paranoiac that was Stalin. He certainly feared for his life, but he also made more enemies than just about any man in history, and betrayed his own allies and loyalists repeatedly. As far as the party leaders go, it appears they feared Stalin, not the masses, and justifiably so. The old revolutionaries had spent their youth and formative years as conspiratorial extremists working underground to overthrow society in a highly repressive environment - perhaps this colored their perceptions. Hard to say.

In my opinion, Stalin was above all a political animal possessed of ruthlessness and pragmatism to see first of all his control of society and the party - which meant a permanent state of terror, and second of all its defense from outside enemies - which meant the implementation of his crash industrialization program. In his view, the easiest way to radically reshape society was terror and repressions and he took that application to it's logical extreme. Would any latent or extant groups opposed to his policy be rounded up in the wide net cast? So much the better: no need for an opposition to even begin to form.